Slightly darken palette_grey_text

[afercia]

Summary

This PR can be summarized in the following changelog entry:

• N/A

Relevant technical choices:

• changes $palette_grey_text from #646464 to #616161 to have a sufficient color contrast ratio on #dddddd backgrounds.

Test instructions

N/A

Fixes #767 Fixes https://github.com/Yoast/my-yoast/issues/132

[boblinthorst]

I can see design agreed with this in one of the underlying issues, and the code is fine, so CR: 👍

[boblinthorst]

As there are no test instructions, and it seems fine, I guess I'll merge it as it is.